The present invention relates to an improved catalyst for use in the partial oxidation of hydrocarbons to prepare dicarboxylic acids and anhydrides. More particularly, the parent invention relates to an improved method of preparing phosphorus-vanadium mixed oxide catalyst. The improved method also produces a superior catalyst which is also the subject matter of the present invention.
Basically, all of the methods and oxidation catalysts for this use seek to obtain vanadium in a valence state of less than +5. One method of achieving this is to begin with vanadium in less than the +5 valence state. Another method and that used most widely in the art is to start with vanadium in the +5 state and reduce the valency to less than +5. This invention relates to the latter method.
Usually the reduced vanadium has been obtained by reducing V.sub.2 O.sub.5 in a solution with HCl to obtain vanadyl chloride. A typical catalyst preparation may involve dissolving the vanadium, phosphorus, and other components in a common solvent, such as hot hydrochloric acid and thereafter depositing the solution onto a carrier. The reduced vanadium with a valence of less than 5 is obtained by initially using a vanadium compound with a valence of plus 5 such as V.sub.2 O.sub.5 and thereafter reducing to the lower valence with, for example, hydrochloric acid during the catalyst preparation to form the vanadium oxysalt, vanadyl chloride, in situ. The vanadium compound is dissolved in a reducing solvent, such as hydrochloric acid, which solvent functions not only to form a solvent for the reaction, but also to reduce the valence of the vanadium compound to a valence of less than 5. For example, a vanadium compound, a copper compound, a tellurium compound, phosphorus compound and alkali metal compound may be dissolved in any order in a suitable reducing solvent and the formation of the complex allowed to take place. Preferably, the vanadium compound is first dissolved in the solvent and thereafter the phosphorus, copper, tellurium and other metal compounds, if any, are added. The reaction to form the complex may be accelerated by the application of heat. The deep blue color of the solution shows the vanadium has an average valence of less than 5. The complex formed is then, without a precipitation step, deposited as a solution onto a carrier and dried. In this procedure, the vanadium has an average valence of less than plus 5, such as about plus 4, at the time it is deposited onto the carrier. Generally, the average valence of the vanadium will be between about plus 2.5 and 4.6 at the time of deposition onto the carrier.
In another method the catalyst is prepared by precipitating the metal compounds, either with or without a carrier, from a colloidal dispersion of the ingredients in an inert liquid. In some instances the catalyst may be deposited as molten metal compounds onto a carrier; however, care must be taken not to vaporize off any of the ingredients such as phosphorus. The catalysts have also been prepared by heating and mixing anhydrous forms of phosphorus acids with vanadium compounds, copper compounds, Me compounds, and the alk-metal compound. The catalysts may be used as either fluid bed or fixed bed catalysts. In any of the methods of preparation, heat may be applied to accelerate the formation of the complex.
A very old and traditional method of obtaining vanadyl chloride as disclosed by Koppel et al, Zeit. anorg. Chem, 45, p. 346-351, 1905 is the reduction of V.sub.2 O.sub.5 in alcoholic HCl solution. This method has been recommended for the preparation of the phosphorus-vanadium oxidation catalyst for example, by Kerr in U.S. Pat. No. 3,255,211 where the solvent also serves as the reducing agent. Subsequently, U.S. Pat. No. 4,043,943 employed this method of reducing vanadium to prepare the basic phosphorus-vanadium catalyst, however, catalyst produced in this manner are known to require a very specific activation procedure in order to be useful as catalyst, as described for example, in U.S. Pat. No. 4,017,521.